Phenolic resins are the reaction product of a phenol and an aldehyde. Phenolic resins are available in two types: novolac resins and resole resins. Novolac resins have a molar excess of the phenolic compound. Novolac resins are not thermosetting resins. Rather, they require a catalyst to cure. Resole resins have a molar excess of the aldehyde compound, and are thermosetting. However, catalysts can be used with resole resins.
Phenolic resins are known in the art, inter alia, as versatile binders for composite materials suitable for many uses. In particular, phenolic resins are used to form materials that can be molded, with or without pressure, to form a desired shape. For example, phenolic resole resins are used as binders to produce structural panels and other products.
Because phenolic resole resins are cured by heat or chemically (by addition of acid, for example), they are particularly suited for molding processes. Molded products can be formed of resin alone, or can include substrates. For example, phenolic resins are used to form parts that are resin alone, and to bind together wood pieces to form composite boards, such as plywood or oriented strand board. Other material, such as fiber and wire, can serve as reinforcement for the resin. Other types of substrates also are known. For example, metal parts may be placed in a mold, which then is filled with resin. The molded piece thus incorporates the metal pieces in the resin.
Resin used for molding is required to satisfy many processing requirements. Phenolic resole resin is used in various processes, such as molding, pultrusion, forming of shaped reinforced objects, and forming of shaped objects with pressure. Whereas some of these processes require only mixing of resin with substrate, then curing, other processes require impregnation of the substrate, which might be in the form of a woven or non-woven mat or fiber. Each of these processes makes different demands on the resin.
Phenolic resole resins often are cured, or advanced, through three stages, specifically, A-stage, B-stage, and C-stage. An A-stage resin is a liquid wherein the reactants are mixed and may have begun to cure, but is not fully cured. A B-stage resin is an A-stage resin that is partially cured or dried. A C-stage resin is a fully cured resin. The time for which a resin can remain in each phase thus may be important to processability of the resin and the product containing it.
Resole resins used in processes that require impregnation of a mat or fiber, such as a process of making a reinforced composite product, require control of the pot life, or gel time of the resin. Pot life denotes the period during which the resin, in whatever stage, is suitable for use in the impregnation and manipulation steps of a process that may include impregnation, manipulation, hardening, and curing steps. Examples of such processes include processes that require saturation of a substrate with resin. Saturation of paper, such as kraft paper for formation of laminates and other papers, such as to form treated filters, is a particular example of such processes. Saturation of a woven or unwoven mat of fibers, such as glass fibers, for manufacture of reinforced structural members and pieces, is another example of such processes. After the impregnation and manipulation steps, the resin may be hardened, or cured.
Some applications require adequate ‘shelf life’ of the resin-impregnated substrate. Such an application requires that, during advancement, the B-staged, impregnated substrate be shelf-stable, i.e., afford the opportunity to keep the resin impregnated into the substrate at the B-stage. Thus, resins that proceed rapidly from A-stage to C-stage are unsuitable for such an application. The ‘shelf life’ is the period during which the resin remains in the B-stage. Thus, it is desired that the resin advance to B-stage quickly and have adequate shelf life at B-stage. Then, when the resin is again advanced, it should harden and cure quickly.
An uncatalyzed phenolic resole resin may provide a pot life sufficient to afford a reasonable opportunity to impregnate a substrate and manipulate the impregnated substrate. However, such a resin requires a long oven time (a long heating time) to advance through B-stage to cure. Such typical heat-reactive resins take too long to cure. Such resins require a long oven time to advance to B-stage, and then to complete cure.
One way to induce hardening is to add a chemical that catalyzes the reaction to induce hardening at the appropriate time. For example, addition of an acid is a known method for inducing curing. Such resins often are called acid-catalyzed resins.
Acid catalysis causes a phenolic resin to cure quickly. Acids used for such resins include the mineral acids, such as hydrochloric acid and sulfuric acid; organic acids, such as oxalic acid and maleic acid; and organic anhydrides, such as maleic anhydride. Resins at low pH in the presence of acid have a very fast cure, and a relatively short pot life. Typically, the pot life of such acid-cured resins is too short for processes that require impregnation and manipulation steps.
Latent catalyst systems were developed in an attempt to ameliorate the rapid cure rate of acid-catalyzed resins. Latent catalyst systems include phosphite hardeners. Latent catalyst systems typically provide a longer pot life than do acid catalyst systems. Indeed, the pot life may be sufficient for impregnation of substrates. However, latent catalyst systems typically cure too quickly upon heating, curing quickly from A-stage to C-stage. Therefore, these resins are unsatisfactory for systems requiring impregnation and storage life for substrate impregnated with resin and advanced to the B-stage.
It is possible to improve the cure speed of a resole resin by increasing the pH. However, to do so may create other problems. Adjusting the pH of a typical phenolic resole resin to 7-8 provides a resin with an appropriate pot life required for impregnation of substrate. However, such a resin cures slowly and typically has a poor shelf life. Hydroxides and organic amines typically are used to increase pH and accelerate hardening in these resins, but at the expense of pot life.
These catalyst systems illustrate the difficulty in producing a phenolic resole resin system that meets the requirements of processes that require reasonable pot time for impregnation and manipulation to a B-stage, yet cure quickly when advancement to the C-stage is desired. Thus, there exists a need for a phenolic resole resin that has a pot life suitable for impregnation of substrate, a long shelf life when partially cured, and a rapid cure rate thereafter.